The present invention relates to a class of bridged metallocene compounds, to the process for their preparation and to the use of these metallocenes as catalyst components for the polymerization of olefins.
Metallocene compounds with two bridged cyclopentadienyl groups are known as catalyst components for the polymerization of olefins.
For example, European Patent Application EP-A-129,368 describes a catalyst system for the polymerization of olefins which comprises a bis-cyclopentadienyl coordination complex with a transition metal, wherein the two cyclopentadienyl groups can be joined by a bridging group.
In this type of metallocene compounds the two cyclopentadienyl groups are generally bridged by divalent radicals having two or more carbon atoms, such as an ethylidene group, or with atoms other than carbon, such. as a dimethylsilanediyl group.
Metallocene compounds having two cyclopentadienyl groups bridged by a single carbon atom are also known. In particular, metallocene compounds of this type having two different cyclopentadienyl groups are known.
For example, European Patent Application EP-A-351,392 describes a catalyst which can be used for the preparation of syndiotactic polyolefins and contains a metallocene compound with two cyclopentadienyl groups linked by a bridge between them, in which one of the two cyclopentadienyl groups is substituted in a manner different from that of the other. The compound indicated as being preferred is isopropylidene(fluorenyl) (cyclopentadienyl)hafnium dichloride.
As regards metallocene compounds having two equally substituted cyclopentadienyl groups bridged by a single carbon atom, in European Patent Application EP 416,566 it is described the polymerization of propylene, carried out in liquid monomer in the presence of a catalyst consisting of (A) an alumoxane and (B) a metallocene compound in which the cyclopentadienyl rings, which can be identical or different, are linked via a bridge of the formula xe2x80x94R5CR6xe2x80x94 in which R5 and R6 can have different meanings. The only compound given as an example is isopropylidene-bis(indenyl)zirconium dichloride. However, the thus obtainable propylene polymers have a very low molecular weight.
I. F. Urazowski et al. at the Xth Fechem Conference on Organometallic Chemistry held on Sep. 5-10, 1993 in Agia Pelagia, Crete-Greece presented metallocene complexes of Ti and Zr obtained from two dicyclopentadienyl-dimethyl-methanes, namely those having an isopropyl or tertbutyl substituent on the 3-position of each cyclopentadienyl ring. However, only mechanisms of the formation of those complexes and their structural features on the basis of X-ray analysis were discussed.
A novel class of metallocene compounds has now been found which has two identical cyclopentadienyl ligands which are linked to one another by an alkylidene bridge and which can advantageously be used as catalyst components for the polymerization of olefins.
An object of the present invention is therefore a metallocene compound of the formula (I): 
wherein R1, R2, R3 and R4, which can be identical or different, are hydrogen atoms or C1-C20-alkyl, C3-C20-cycloalkyl, C2-C20-alkenyl, C6-C20-aryl, C7-C20-alkylaryl or C7-C2-aryl-alkyl groups which can contain silicon or germanium atoms, R3 being different from R2 and from a hydrogen atom, and wherein R1 and R2 on the same cyclopentadienyl ring can form a ring having 5 to 8 carbon atoms;
R5 is a hydrogen atom or a xe2x80x94CHR7R8 group;
R6 is a C6-C20-aryl radical or a xe2x80x94CHR9R10 group;
R5 and R6 can form a ring having 3 to 8 carbon atoms which can contain hetero atoms;
R7, R8, R9 and R10, which can be identical or different, are hydrogen atoms or C1-C20-alkyl, C3-C20-cycloalkyl, C2-C20-alkenyl, C6-C20-aryl, C7-C20-alkylaryl or C7-C20-arylalkyl radicals which can contain hetero atoms such as nitrogen, phosphor, oxygen or sulphur, and two R7, R8, R9 and R10 substituents can form a ring having 3 to 8 carbon atoms which can contain hetero atoms;
M is an atom of a transition metal selected from those belonging to group 3, 4, 5, 6 or to the lanthanide or actinide groups in the Periodic Table of the Elements (new IUPAC version);
the X substituents, which can be identical or different, are hydrogen atoms, halogen atoms or R, OR, SR, NR2 or PR2 groups, wherein the R substituents are C1-C20-alkyl, C3-C20-cycloalkyl, C2-C20-alkenyl, C6-C20-aryl , C7-C20-alkylaryl or C7-C20-arylalkyl radicals which can contain silicon or germanium atoms;
with the proviso that, when the R1, R2 and R4 substituents are hydrogen atoms and the R5 and the R6 substituents are methyl groups, then the R3 substituents are other than an isopropropyl or tertbutyl group.
The transition metal M is preferably selected from titanium, zirconium and hafnium and, more preferably, is zirconium.
The X substituents are preferably chlorine atoms or methyl radicals.
A particularly interesting class of metallocenes according to the invention is that of the compounds of the formula (I) in which the R2 substituents are hydrogen atoms. The R1 substituents are preferably different from hydrogen atoms. The R3 substituents are preferably carbon, silicon or germanium atoms substituted with three alkyl, cycloalkyl, aryl, alkylaryl or arylalkyl groups having 1 to 10 carbon atoms. The R4 substituents are preferably hydrogen atoms. Non-limiting examples of metallocene compounds belonging to this class are:
isopropylidene-bis(3-methyl-cyclopentadienyl)zirconium dichloride,
isopropylidene-bis(3-isopropyl-cyclopentadienyl)zirconium dichloride,
isopropylidene-bis(3-t-butyl-cyclopentadienyl)zirconium dichloride,
isopropylidene-bis(2,4-dimethyl-cyclopentadienyl)zirconium dichloride,
isopropylidene-bis(2-methyl-4-t-butyl-cyclopentadienyl)zirconium dichloride and
isopropylidene-bis(2-methyl-4-phenyl-cyclopentadienyl)zirconium dichloride.
Another particularly interesting class of metallocenes according to the invention is that of the compounds of the formula (II): 
and the corresponding bis-4,5,6,7-tetrahydroindenyl compounds, wherein R3, R4, R5, R6, M and X are defined as above, and the six-carbon-atom rings of the indenyl ligands can optionally be substituted. The R3 substituents are preferably carbon, silicon or germanium atoms substituted with three alkyl, cycloalkyl, aryl, alkylaryl or arylalkyl groups having 1 to 10 carbon atoms. The R4 substituents are preferably hydrogen atoms. Non-limiting examples of metallocene compounds belonging to this class are:
isopropylidene-bis(3-methyl-indenyl)zirconium dichloride,
isopropylidene-bis(3-ethyl-indenyl)zirconium dichloride,
isopropylidene-bis(3-isopropyl-indenyl)zirconium dichloride,
isopropylidene-bis(3-t-butyl-indenyl)zirconium dichloride,
isopropylidene-bis(3-trimethylsilyl-indenyl)zirconium dichloride,
isopropylidene-bis(3-trimethylgermyl-indenyl)zirconium dichloride,
isopropylidene-bis(3-t-butyl-4,5,6,7-tetrahydroindenyl)zirconium dichloride.
The metallocene compounds of the formula (I) can be prepared by a process which represents another object of the present invention and which comprises the reaction of the corresponding bis-cyclopentadienyl ligands of the formula (III): 
wherein R1, R2, R3, R4, R5 and R6 are defined as above, and A is a suitable leaving group, with a compound of the formula MX4, wherein M and X are defined as above. The double bonds of the cyclopentadienyl rings in the ligands of formula (III) can be in any of the allowed positions. The ligands of the formula (III) can be prepared, for example, by the method described in the co-pending Italian Patent Application No. MI/95A/100 in the name of the same Applicant.
In the case in which at least one substituent X in the metallocene compound of the formula (I) which is to be. prepared is other than a halogen, it is necessary to substitute at least one substituent X in the metallocene obtained by at least one substituent X other than a halogen.
The reaction of substituting substituents X by substituents X other than a halogen is carried out using generally applied methods. For example, if the desired substituents X are alkyl groups, the metallocenes can be made to react with alkylmagnesium halides (Grignard reagents) or with alkyllithium compounds.
The metallocene compounds of the present invention can conveniently be used as catalyst components for the polymerization of olefins.
Still another object of the present invention is therefore a catalyst for the polymerization of olefins, consisting of the product of the reaction between:
(a) a metallocene compound according to the invention, and
(b) an alumoxane or a compound able to form an alkylmetallocene cation.
In the catalyst used in the process according to the invention, both the metallocene compound of the formula (I) and the alumoxane can be present as the product of the reaction with an organometallic aluminium compound of the formula AlR113 or Al2R116, in which the substituents R11 which can be identical or different are defined as for the substituents R or are halogen atoms.
The alumoxane used in the catalyst according to the invention is a linear, branched or cyclic compound containing at least one group of the type: 
wherein the substituents R12 which can be identical or different are defined as for the substituent R or are a group xe2x80x94Oxe2x80x94Al(R12)2 and, if appropriate, some R12 can be halogen atoms.
In particular, alumoxanes of the formula: 
can be used in the case of linear compounds, wherein n is 0 or an integer of between 1 and 40 and the substituents R12 are defined as for the substituents R, or alumoxanes of the formula: 
can be used in the case of cyclic compounds, with n being an integer of between 2 and 40 and the substituents R12 being defined as for the substituents R.
The substituents R12 are preferably methyl, ethyl, isobutyl or 2,4,4-trimethyl-pentyl.
Examples of alumoxanes suitable for use according to the present invention are methylalumoxane (MAO), isobutylalumoxane (TIBAO) and 2,4,4-trimethyl-pentylalumoxane (TIOAO).
Non-limiting examples of aluminium compounds of the formula AlR113 or Al2R116 are:
Al(Me)3, Al(Et)3, AlH(Et)2, Al(iBu)3, AlH(iBu)2, Al(iHex)3, Al(iOct)3, Al(C6H5)3, Al(CH2C6H5)3, Al(CH2CMe3)3, Al(CH2SiMe3)3, Al(Me)2iBu, Al(Me)2Et, AlMe(Et)2, AlMe(iBu)2, Al(Me)2iBu, Al(Me)2Cl, Al(Et)2Cl, AlEtCl2 and Al2(Et)3Cl3, wherein Me=methyl, Et=ethyl, iBu=isobutyl and iHex=isohexyl, ioct=2,4,4-trimethyl-pentyl.
Amongst the above aluminium compounds, trimethylaluminium (TMA) and triisobutylaluminium (TIBAL) are preferred.
Non-limiting examples of compounds able to form an alkylmetallocene cation are compounds of the formula Y+Zxe2x88x92, wherein Y+ is a Brxc3x6nsted acid, able to donate a proton and to react irreversibly with a substituent X of the compound of the formula (I), and Zxe2x88x92 is a compatible anion which does not coordinate and which is able to stabilize the active catalytic species which results from the reaction of the two compounds and which is sufficiently labile to be displaceable by an olefin substrate. Preferably, the anion Zxe2x88x92 consists of one or more boron atoms. More preferably, the anion Zxe2x88x92 is an anion of the formula BAr4(xe2x88x92), wherein the substituents Ar which can be identical or different are aryl radicals such as phenyl, pentafluorophenyl or bis(trifluoromethyl)phenyl. Tetrakispentafluorophenyl borate is particularly preferred. Moreover, compounds of the formula BAr3 can conveniently be used.
The catalysts of the present invention can also be used on inert supports. This is achieved by depositing the metallocene compound (A) or the product of the reaction thereof with the component (B), or the component (B) and then the metallocene compound (A) on inert supports such as, for example, silica, alumina, styrene/divinylbenzene copolymers or polyethylene.
The solid compound thus obtained, in combination with the further addition of the alkylaluminium compound either as such or prereacted with water if necessary, is usefully employed in gas-phase polymerization.
A further object of the present invention is a process for the polymerization of olefins, which comprises the polymerization reaction of one or more olefin monomers in the presence of a catalyst as described above.
Preferred olefin monomers are ethylene, the xcex1-olefins and the cycloolefins. The catalysts according to the invention can conveniently be used, for instance, in the homopolymerization reactions of ethylene or of xcex1-olefins such as propylene and 1-butene, in the copolymerization reactions of ethylene with xcex1-olefins such as propylene and 1-butene, and also in the copolymerization reactions of propylene with C4-C10 xcex1-olefins such as 1-butene. Particularly interesting results are achieved when the catalysts of the invention are used for the polymerization of propylene.
Thus, according to an embodiment of the process for olefin polymerization of the invention, propylene is polymerized in the presence of a metallocene compound of the formula (II), wherein the R3 substituents are carbon, silicon or germanium atoms substituted with three alkyl, cycloalkyl, aryl, alkylaryl or arylalkyl groups having 1 to 10 carbon atoms, and wherein R4, R5, R6, M and X are defined as above. The R4 substituents are preferably hydrogen atoms. Examples of those metallocene compounds are:
isopropylidene-bis(3-t-butyl-indenyl)zirconium dichloride,
isopropylidene-bis(3-trimethylsilyl-indenyl)zirconium dichloride, and
isopropylidene-bis(3-trimethylgermyl-indenyl)zirconium dichloride.
The thus obtainable propylene polymers have narrow molecular weight distributions coupled with high isotactic indexes and a very high levels of regioregularity. In fact, the 13C-NMR analysis carried out on these polymers does not show structural units due to regioirregular insertions. Reference is made to xe2x80x9cMacromolecules, 1995, vol.28, pagg. 6667-6676xe2x80x9d.
Thus, another object of the present invention is a propylene homopolymer having the following characteristics:
molecular weight distribution (Mw/Mn) lower than 4, preferably lower than 3.5, more preferably lower than 3,
isotactic (mmmm) pentads, as determined by 13C-NMR analyses, higher than 70%, preferably comprised between 75 and 97%, more preferably between 80 and 95%,
no structural units due to regioirregular insertions detectable at the 13C-NMR analysis carried out with a 300 MHz instrument.
If the polymerization of propylene is carried out in the presence of a bis-4,5,6,7-tetrahydroindenyl metallocene compound corresponding to the above said compounds of the formula (II), a very low molecular weight polypropylene wax is obtained. Notwithstanding the low molecular weight, these waxes have fairly high isotactic indexes as demonstrated by the presence of a melting point and by the values of isotactic (m) diads, as determined by 13C-NMR analyses, which are generally higher than 90%.
According to another embodiment of the process for olefin polymerization of the invention, propylene is polymerized in the presence of a metallocene compound of the formula (I) in which the R2 substituents are hydrogen atoms and the R3 substituents are carbon, silicon or germanium atoms substituted with three alkyl, cycloalkyl, aryl, alkylaryl or arylalkyl groups having 1 to 10 carbon atoms. The substituents are preferably hydrogen atoms. Examples of the metallocene compounds are:
isopropylidene-bis(3-t-butyl-cyclopentadienyl)zirconium dichloride, and
isopropylidene-bis(2-methyl-4-t-butyl-cyclopentadienyl)zirconium dichloride.
The thus obtainable propylene polymers, besides narrow molecular weight distributions, have very high isotactic indexes, as results from their high melting points which are generally higher than 155xc2x0 C. and also higher than 160xc2x0 C.
Thus, a further object of the present invention is a propylene homopolymer having the following characteristics:
molecular weight distribution (Mw/Mn) lower than 4, preferably lower than 3.5, more preferably lower than 3,
isotactic (m) diads, as determined by 13C-NMR analyses higher than 99%, preferably higher than 99.5%.
Even the values of isotactic (mmmm) pentads for these polymers can be as high as 99% and over.
These polymers do not have a very high level of regioregularity. In fact, the 13C-NMR analysis carried out with a 300 MHz instrument on these polymers generally show the presence of a low amount of structural units due to regioirregular insertions, such as 1,3 insertions.
Particularly interesting results are achieved when in the above said specific metallocene compounds of the formula (I) the R1 substituents are different from hydrogen atoms, such as for the isopropylidene-bis(2-methyl-4-t-butyl-cyclopentadienyl)zirconium dichloride. It is thus possible to obtain propylene polymers having very high isotactic indexes, as results from their melting points which can be higher than 160xc2x0 C. even at polymerization temperatures of industrial interest, such as 50xc2x0 C. and higher.
The propylene polymers obtainable from the process of the invention have low xylene-soluble fractions, generally lower than 5% by weight, preferably lower than 3% by weight, more preferably lower than 1% by weight.
The polymerization reaction of propylene according to the invention can be carried out in the presence of a C4-C10 xcex1-olefin comonomer, such as 1-butene. It is thus possible to obtain propylene copolymers with 0.1-10% by moles of a C4-C10 xcex1-olefin comonomer having characteristics similar to those of the corresponding homopolymer but a lower melting point. Notwithstanding the presence of a comonomer, these copolymers still have an extremely low xylene-soluble fractions, generally lower than 3% by weight, preferably lower than 2% by weight, more preferably lower than 1% by weight.
Thus, a still further object of the present invention is a propylene copolymers with 0.1-10% by moles of a C4-C10 xcex1-olefin comonomer, preferably 1-butene, having the following characteristics:
isotactic (m) diads, as determined by 13C-NMR analyses, higher than 70%, preferably higher than 75%, more preferably higher than 80%,
molecular weight, distribution (Mw/Mn) lower than 4, preferably lower than 3.5, more preferably lower than 3,
xylene-soluble fractions lower than 3% by weight, preferably lower than 2% by weight, more preferably lower than 1% by weight.
The process for the polymerization of olefins according to the invention can be carried out in the liquid phase in the presence or absence of an inert hydrocarbon solvent, or in the gas phase. The hydrocarbon solvent can either be aromatic such as toluene, or aliphatic such as propane, hexane, heptane, isobutane or cyclohexane.
The polymerization temperature is generally comprised between xe2x88x92100xc2x0 C. and +80xc2x0 C., and more particularly between xe2x88x9250xc2x0 C. and +50xc2x0 C. The lower the polymerization temperature, the higher are the resulting molecular weights of the polymers obtained.
The molecular weight of the polymers can be also varied by varying the type or the concentration of the catalyst components or using molecular weight regulators such as, for example, hydrogen.
The molecular weight distribution can be varied by using mixtures of different metallocene compounds or by carrying out the polymerization in several stages at different polymerization temperatures and/or different concentrations of the molecular weight regulators.
The polymerization yields depend on the purity of the metallocene compound of the catalyst. The metallocene compounds obtained by the process of the invention can therefore be used as such or can be subjected to purification treatments.
The components of the catalyst can be put into contact with one another before the polymerization. The contact time is generally between 1 and 60 minutes, preferably between 5 and 20 minutes. The pre-contact concentrations are between 10xe2x88x922 and 10xe2x88x928 mol/l for the metallocene component (A), while they are between 10 and 10xe2x88x923 mol/l for the component (B). The pre-contact is generally effected in the presence of a hydrocarbon solvent and, if appropriate, of small quantities of monomer.